As mobile devices have been increasingly developed, and the demand for such mobile devices has increased, the demand for secondary batteries has also sharply increased as an energy source for the mobile devices. Among them is a lithium secondary battery having high energy density and high discharge voltage, on which much research has been carried out and which is now commercially and widely used.
Based on the appearance thereof, the lithium secondary battery may be classified as a cylindrical battery, a prismatic battery, or a pouch-shaped battery. Based on the kind of an electrolyte used therein, the lithium secondary battery may be also classified as a lithium-ion battery or a lithium-ion polymer battery. A recent trend in the miniaturization of mobile devices has increased demand for the prismatic battery or the pouch-shaped battery, which have a small thickness.
However, various combustible materials are contained in the lithium secondary battery. As a result, there is a possibility of danger in that the lithium secondary battery will be heated or explode due to overcharge, overcurrent, or any other external physical impacts. In other words, the lithium secondary battery has low safety. Consequently, when the lithium secondary battery is exposed to high temperature, or when a large amount of current flows in the battery within a short time due to overdischarge, an external short circuit, a nail penetration, or a local crush, the battery is heated due to IR heat generation, with the result that the battery may catch fire or explode.
As the temperature of the battery increases, the reaction between an electrolyte and electrodes is accelerated. As a result, heat of reaction is generated, and therefore, the temperature of the battery further increases, which accelerates the reaction between the electrolyte and the electrodes. Consequently, the temperature of the battery sharply increases, and therefore, the reaction between the electrolyte and the electrodes is further accelerated. This vicious cycle causes a thermal runaway phenomenon in which the temperature of the battery sharply increases. When the temperature of the battery increases to a predetermined temperature level, the battery may catch fire. Also, as a result of the reaction between the electrolyte and the electrodes, gas is generated, and therefore, the internal pressure of the battery increases. When the internal pressure of the lithium secondary battery increases to a predetermined pressure level, the battery may explode. This possibility that the lithium secondary battery will catch fire or explode is the most fatal disadvantage of the battery.
Therefore, a point to be essentially considered in developing the lithium secondary battery is to secure the safety of the battery. Securing the safety of the battery may be achieved by a method of mounting a safety device outside a cell or a method of using a material contained in the cell. The use of a positive temperature coefficient (PTC) element and a circuit interruption device (CID) element using the change in temperature of the battery, a protection circuit for controlling voltage and current of the battery, and a safety vent using the change in internal pressure of the battery belongs to the former. The addition of a material that is changeable physically, chemically, and electrochemically depending upon the change in internal temperature or voltage of the battery belongs to the latter.
In the former case, a battery pack is constructed in a structure in which a PCM, serving to secondarily protect a battery and having an external input and output terminals connectable to an external device, is connected to a cathode terminal and an anode terminal via nickel plates, and a PTC element, serving to primarily protect the battery from overcurrent, overdischarge, and overcharge of the battery cell, is connected to the electrode terminals. The nickel plates are attached to the top and bottom of the PTC element. The nickel plate attached to the top of the PTC element is electrically connected to the PCM, and the nickel plate attached to the bottom of the PTC element is electrically connected to the electrode terminals of the battery cell.
In order to assemble the battery pack with the above-stated construction, however, several welding processes are required to achieve the electrical connection between the PTC element and the PCM and between the PTC element and the electrode terminals. Furthermore, when welding is performed in a state in which the PTC element and the nickel plates are not in position, a short circuit may occur, or connection regions may separate from each other during the assembly or the use of the battery pack. Also, since the PTC element is connected to the PCM and the battery cell, it is required for the nickel plates to have a large length. The long nickel plates must be bent such that the PCM is loaded on the battery cell, with the result that a dead space corresponding to the bent space is formed, and therefore, the volume density of the battery pack relatively decreases. Furthermore, stress is generated at the time of bending the nickel plates, with the result that the nickel plates or the PTC may crack.
In the latter case, a method of adding an additive for improving safety to the electrolyte or the electrodes may be used as a method of using a material contained in the cell. A chemical safety device has an advantage in that the chemical safety device does not require an additional process and space, and is applicable to all kinds of batteries. However, the chemical safety device has a problem in that the performance of the battery may deteriorate due to the addition of the material. Such material has been reported to include a material to form immovable film at the electrodes and a material to increase the resistance of the electrodes through the increase in volume thereof when temperature increases. However, when the immovable film is formed, a by-product is also formed, with the result that the performance of the battery may deteriorate. Also, the volume of the material occupying the interior of the battery increases, with the result that the capacity of the battery decreases. Consequently, secure safety is not guaranteed when adding the material, and therefore, it is not possible to use a method of adding the material as an independent measure.